1. Field of the Invention
An embodiment presented below relates to a power supply apparatus and a power line communication apparatus suitable as a power line communication apparatus that performs communication over a power line as a signal transmission line.
2. Description of Related Art
To perform wired data communication at home, office, or factory using a terminal, such as a computer, for example, it is normally required to install wiring for cables and connectors used as transmission lines at required locations. A variety of installation works are thus required before starting operation of communication equipment.
Meanwhile, a commercial power supply of, for example, 120 VAC (60 Hz) in the United States of America or 100 VAC (50/60 Hz) in Japan is mostly used at home, office, factory, and the like. Thus, power lines that supply the power are already installed across home, office, factory, and the like. Using the power lines for data communication would eliminate additional installation of wiring exclusively for communication use. That is, simply plugging a communication apparatus into a power outlet allows securing of a communication path.
As power line communication (PLC) technology for communication using a power line, a technology disclosed in Japanese Patent Laid-open Publication 2000-165304 is known, for example. Various manufacturers have been conducting research and development in certain frequencies (e.g., a high frequency, such as 1.705 to 80 MHz in the United States of America, 2 MHz-30 MHz in Japan, or an ultra high frequency). More specifically, it is envisioned that a multi-carrier signal is generated using a plurality of sub-carriers and transmitted on the power line in an OFDM (Orthogonal Frequency Division Multiplexing) system and the like.
FIG. 20 is a block diagram illustrating a configuration example of a power line communication apparatus. The power line communication apparatus includes coupler 910, modem 920, switching regulator 930, noise reduction circuit 940, and signal reduction circuit 950. Coupler 910 has coupled transformer T1, wherein capacitors C1 and C2 that exclude a component of commercial power supply are connected in series to a pair of transmission lines (power lines) and wherein a primary winding is connected in series with capacitors C1 and C2. A signal is transmitted between the transmission lines and modem 920 via coupled transformer T1. A value is selected for capacitors C1 and C2 so as to have high impedance to commercial power supply and low impedance to a transmission signal (a communication signal).
Modem 920 has transmitter 921, receiver 922, and data processor 923. Noise reduction circuit 940 has capacitors C3 and C4 and common mode coil T2. Signal reduction circuit 950 has normal mode coils L1 and L2.
When there is a part or an apparatus that generates noise, such as switching regulator 930 that supplies power to operate the power line communication apparatus or a switching regulator of a peripheral device connected to the same power lines, noise reduction circuit 940 is provided to the power source unit or the peripheral device so as to prevent the noise from flowing to the power lines. Noise reduction circuit 940 is normally configured to lower impedance between the two power lines in order to eliminate or reduce noise. In power line communication that uses the power lines as the transmission lines, however, noise reduction circuit 940 reduces the impedance between the power lines and thus increases signal loss. In order to prevent attenuation of a signal as being affected by the power source unit that drives the power line communication apparatus or the connected peripheral circuit or peripheral device, signal reduction circuit 950 is provided between noise reduction circuit 940 and the power lines.
Noise reduction circuit 940 provided to the switching regulator and the like generally has capacitors (across capacitors) C3 and C4 and common mode coil:(transformer) T2. Capacitors C3 and C4, which are inserted between two lines, cancel normal mode noise, whereas common mode coil T2 cancels common mode noise. Capacitors C3 and C4 shunt the two lines and reduce a normal mode noise between which the two lines' phase is reverse (hereinafter referred to as “anti-phase normal mode noise”). Inductance of common mode coil T2 reduces a common mode noise between which the two lines' phase is the same (hereinafter referred to as “in-phase normal mode noise”).
In power line communication performed on the power lines as the transmission lines, a transmission signal is inserted between the two lines in anti-phase normal mode for communication. Therefore, capacitors C3 and C4, which are inserted between the two lines of noise reduction circuit 940, shunt and attenuate the transmission signal in a frequency band thereof. Capacitors C3 and C4 have substantially low impedance in a frequency band used for power line communication of, for example, 4 MHz to 30 MHz. Noise reduction circuit 940 thereby terminates at low impedance and attenuates a high-frequency signal, which is the transmission signal.
Common mode coil T2 functions as an inductor in in-phase common mode between the two lines. Since the transmission signal output to the transmission lines is in normal mode, however, common mode coil T2 does not function as the inductor for the transmission signal and remains as if not inserted. Further, a frequency for reduction of common mode coil T2 is generally low compared to the transmission signal, and the inductor of common mode coil T2 is designed large. Thus, parallel capacitance is large between the windings (between the transmission lines). Affected by the parallel capacitance that exists as stray capacitance in a frequency band of the transmission signal, common mode coil T2 does not contribute to reduction of the signal. Further, capacitor C4 or switching regulator 930 is taken as a load for the transmission signal, thus leading to attenuation of the transmission signal.
On the contrary, signal reduction circuit 950 is generally formed of normal mode coils L1 and L2 that have a large value, so as to obtain sufficiently higher impedance than the transmission line impedance. The transmission signal is inserted between the two lines in anti-phase (normal mode). Thus, normal mode coils L1 and L2 inserted to the two lines respectively prevent the transmission signal from being leaked to a switching regulator 930 side, being absorbed in noise reduction circuit 940, and being attenuated.
In power line communication that uses the transmission signal in a wideband of, for example, 4 MHz to 30 MHz, the normal mode coils used in the signal reduction circuit are required to maintain high impedance in the wideband. However, it is difficult to make an inductor that constantly has high impedance in a wide signal range. The inductor would be substantially large even when achieved.
In addition, the stray capacitance, including the parallel capacitance component of the inductor between the transmission lines, hampers wideband use. Particularly, a high-power-consumption device, such as the power source unit and the connected peripheral device, has a large inductor, thus causing large stray capacitance and significantly hampering wideband use.
Further, in a case when installed in the high-power-consumption device and in other cases, setting a large value so as to obtain high impedance causes magnetic saturation of a core and the like, and thus it is difficult to-obtain the high impedance.
In a configuration where the signal reduction circuit and the noise reduction circuit are connected in parallel to the coupler and the modem of the power line communication apparatus as described above, the normal mode coil that has high impedance in the wideband is required as the signal reduction circuit. It is difficult, however, to achieve such signal reduction circuit, particularly when the high-power-consumption device is connected.